1. Field of the Invention
The present invention relates to a stainless steel product having low contact electrical resistance, and to a method for producing the stainless steel product. The invention also relates to a bipolar plate produced from the stainless steel product and to a polymer electrode fuel cell (hereinafter may be abbreviated as PEFC) containing the bipolar plate.
2. Description of the Related Art
Stainless steel has excellent corrosion resistance due to passive film formed on the surface thereof. However, stainless steel is not suitable for producing electrically conductive elements requiring low contact electrical resistance, since the passive film formed on the surface has high electrical resistance. In general, the more excellent the corrosion resistance of passive film, the higher the electrical resistance thereof.
Therefore, reduction in contact electrical resistance of stainless steel enables stainless steel to serve as an electrically conductive element such as a terminal, the element being employed in a circumstance requiring corrosion resistance. One example of an electrically conductive element exhibiting excellent corrosion resistance and low contact electrical resistance is a bipolar plate (also called a xe2x80x9cseparatorxe2x80x9d) of a PEFC.
A fuel cell generates DC power, and examples of fuel cells include a solid oxide fuel cell (abbreviated as SOFC), a molten carbonate fuel cell (abbreviated as MCFC), and a phosphoric acid fuel cell (abbreviated as PAFC). These fuel cells are named after a component material of an electrolyte, which is the most important portion of a fuel cell.
At present, fuel cells which have attained a commercially satisfactory level include a PAFC and an MCFC.
Approximate operation temperatures of an SOFC, an MCFC, a PAFC, and a PEFC are 1000xc2x0 C., 650xc2x0 C., 200xc2x0 C., and 80xc2x0 C., respectively.
A PEFC operates at approximately 80xc2x0 C. and is easy to start and stop. The expected energy efficiency thereof is approximately 40%. Therefore, there is worldwide demand for PEFCs, which can be employed practically in an on-site power source used in a small-scale power plant, a telephone office, or a similar site; a domestic small on-site power source making use of city gas as a fuel; and a power source incorporated in a low-pollution electric automobile making use of hydrogen, methanol, or gasoline as a fuel.
Although the aforementioned fuel cells are categorized as fuel cells, i.e., their names include the term xe2x80x9cfuel cell,xe2x80x9d they must be considered individually when a component material of a fuel cell is designed, since performance required for a component material, particularly anti-corrosion performance, varies with the type of fuel cell.
Specifically, the performance depends on corrosion of a component material caused by an employed electrolyte;
oxidation at high temperature predominantly occurring above approximately 380xc2x0 C.; and sublimation and re-deposition of an electrolyte, and condensation.
In practice, a variety of materials are employed as component materials of a fuel cell; e.g., graphite materials, Ni cladding, alloys having a high alloying element content, and stainless steel.
Thus, materials per se employed in commercialized PAFCs and MCFCs cannot be applied to a component material of PEFCs.
FIGS. 1A and 1B shows the structure of a PEFC; i.e., FIG. 1A is an exploded view of a fuel cell (membrane electrode assemblies) and FIG. 1B is a perspective view of an entire fuel cell. As shown in FIGS. 1A and 1B, a fuel cell 1 is an assembly of membrane electrode assemblies. The membrane electrode assembly comprises a solid polymer electrolyte membrane 2, a fuel electrode (anode) membrane 3 being laminated on one surface of the solid polymer electrolyte membrane 2 and an oxidizing agent electrode (cathode) membrane 4 being laminated on the other surface. The membrane 3 is further layered with a bipolar plate 5a, while the membrane 4 is further layered with a bipolar plate 5b. 
The solid polymer electrolyte membrane 2 comprises a proton-conductive fluoride membrane having a hydrogen-ion (proton)-exchange group.
Each of the anode membrane 3 and the cathode membrane 4 is provided with a catalyst layer comprising a granular platinum catalyst, graphite powder, and an optional fluororesin having a hydrogen-ion (proton)-exchange group, which is to come into contact with a fuel gas or an oxidizing gas.
A fuel gas A (hydrogen or a hydrogen-containing gas) is fed through channels 6a provided in the bipolar plate 5a, to thereby supply hydrogen to the anode membrane, while an oxidizing gas B such as air is fed through channels 6b provided in the bipolar plate 5b, to thereby supply oxygen. The thus-supplied gasses induce electrochemical reaction, to thereby generate DC power.
Functions required of a bipolar plate of a PEFC are as follows:
(1) a function of a channel which supplies a fuel gas and an oxidizing gas uniformly in inner planes of a cell;
(2) a function of a channel which effectively discharges water formed in cathode portions to outside a fuel cell along with a carrier gas such as air or oxygen after reaction;
(3) a function of an electrical connector between membrane electrode assemblies so as to maintain low resistance and high conductivity suitable for an electrode for a long period of time;
(4) a function of a separator which separates a cathode chamber and an anode chamber in adjacent assemblies; and
(5) a function of a separator which isolates cooling water channels and separates adjacent assemblies.
Hitherto, there has been earnestly investigated application of a carbon sheet as a material of a bipolar plate of a PEFC. However, a carbon sheet is disadvantageous in that the sheet is easily fractured and severely elevates cost of mechanical processing for producing a flat surface and forming gas channels. These fatal problems might make commercialization of a fuel cell difficult.
Among carbonaceous materials, thermally expandable graphite has been most attractive material for producing a bipolar plate of a PEFC, in that the graphite is considerably inexpensive. However, in order to provide functions of the aforementioned separators by means of reducing gas-permeability, thermally expandable graphite must be subjected to a plurality of steps of resin impregnation and firing. In addition, there still remain problems in cost of mechanical processing for ensuring surface flatness and forming channels. Thus, commercialization of thermally expandable graphite has not yet been attained.
In contrast to investigation of application of graphite materials, stainless steel has been applied to a bipolar plate, in view of cost reduction.
Japanese Patent Application Laid-Open (kokai) No. 10-228914 discloses a fuel cell bipolar plate which is formed of a metallic material, in which a surface of the bipolar plate which contacts with a membrane electrode assembly is plated directly with gold. Examples of metallic materials include stainless steel, aluminum, and Ni-Fe alloy, with Type 304 being employed as stainless steel. According to the disclosure, the bipolar plate is plated with gold, to thereby lower contact resistance between the bipolar plate and an electrode and enhance electric conduction from the bipolar plate to the electrode. Thus, a fuel cell containing such bipolar plates is considered to generate high output power.
Japanese Patent Application Laid-Open (kokai) No. 8-180883 discloses a PEFC employing bipolar plates formed of a metallic material which is easily coated with passive film in air. According to the disclosure, the metallic surface of the bipolar plates is completely coated with passive film, to thereby make the surface resistant to chemical substances. Thus, ionization of water formed in the fuel cell is suppressed, to thereby suppress lowering efficiency of electrochemical reaction. It is also disclosed that passive film on a portion contacting with an electrode membrane of a bipolar plate is removed and a layer of a noble metal is formed, to thereby lower contact electrical resistance.
However, even though the disclosed metallic materials such as stainless steel coated with passive film per se are employed, bipolar plates produced from the materials exhibit poor corrosion resistance and release metal ions. The released metal ions form corrosion products such as chromium hydroxide and iron hydroxide, to thereby disadvantageously elevate contact electrical resistance of the bipolar plates. Thus, at present, bipolar plates are plated with a noble metal such as gold, despite the cost thereof.
In view of the foregoing, an object of the present invention is to provide stainless steel products having low contact electrical resistance for producing an electricity-conducting element. Another object of the invention is to provide a bipolar plate formed of the stainless steel product. Still another object of the invention is to provide a PEFC comprising the bipolar plate formed of the stainless steel product.
Accordingly, the present invention is directed to the followings.
(1) A stainless steel product exhibiting low contact electrical resistance, wherein at least one of a conductive metallic inclusion of carbide and a conductive metallic inclusion of boride protrudes through an outer surface of passive film from stainless steel under the passive film.
(2) A bipolar plate for fabricating a polymer electrode fuel cell, which bipolar plate comprises a stainless steel product as recited in (1).
(3) A method for producing a stainless steel product which comprises corroding the surface of a stainless steel product by use of an aqueous acidic solution to thereby expose at least one of a conductive metallic inclusion of carbide and a conductive metallic inclusion of boride on the surface; neutralizing the product by use of an aqueous alkaline solution having a pH of 7 or more; and washing and drying the product.
(4) A polymer electrode fuel cell in which a fuel gas and an oxidizing agent gas are supplied to an assembly produced by laminating a plurality of membrane electrode assemblies while inserting a bipolar plate between membrane electrode assemblies to thereby generate DC power, which fuel cell has a bipolar plate as recited in (2).
As used herein, the term xe2x80x9ca conductive metallic inclusion of carbidexe2x80x9d refers to as a metallic inclusion of carbide such as M23C6, M4C, M2C, MC or any mixture thereof and the term xe2x80x9ca conductive metallic inclusion of boridexe2x80x9d refers to a metallic inclusion of boride such as M2B. The symbol xe2x80x9cMxe2x80x9d represents a metallic element, which is not limited to a specific metal and can be any metal exhibiting strong affinity with C or B. Typically, M predominantly comprises Cr and Fe and contains microamounts of Ni and Mo. Examples of M23C6 metallic inclusions include Cr23C6 and (Cr, Fe)23C6. Examples of M2C metallic inclusions include Mo2C. Examples of MC metallic inclusions include WC. Examples of M2B metallic inclusions include Cr2B, (Cr, Fe)2B, (Cr, Fe, Ni)2B, (Cr, Fe, Mo)2B, and (Cr, Fe, Ni, Mo)2B, Cr1.2Fe0.76Ni0.04B. Examples of M4B metallic inclusions include B4C. In principle, any metallic inclusion exhibiting excellent electrical conductivity may exhibit similar performance.
The subscript xe2x80x9c2xe2x80x9d in xe2x80x9cM2Bxe2x80x9d refers to a stoichiometric coefficient represented by [(Cr mass %/Cr atomic weight)+(Fe mass %/Fe atomic weight)+(Mo mass %/Mo atomic weight)+(Ni mass %/Ni atomic weight)+(X mass %/X atomic weight)]/(B mass %/B atomic weight) of approximately 2, wherein X represents a metal element other than Cr, Fe, Mo, and Ni. This style of expression is not specific, but very general.
In addition, a metallic inclusion of carbide such as M23C6, M4C, M2C, or MC or a metallic inclusion of boride such as M2B also encompasses a metallic inclusion precipitated in the form as described below.
Specifically, although C and B in the aforementioned metallic inclusions represent carbon and boron, the two elements may substitute for each other. For example a metallic inclusion of carbide such as M23(C, B)6, M4(C, B), M2(C, B), or M(C, B) and a metallic inclusion of boride such as M2(B, C) may be precipitated. Furthermore, a metallic inclusion of carbide such as M23C6, M4C, M2C, or MC and a metallic inclusion of boride such as M2B may be co-precipitated together instead of individually.
Thus, in the present invention, even though the metallic inclusion of carbide such as M23C6, M4C, M2C, or MC or the metallic inclusion of boride such as M2B may take any chemical form, these inclusions exhibiting excellent electrical conductivity are dispersed to thereby exhibit excellent performance.
In general, a bipolar plate has the below-described five functions:
a) a function of a channel which supplies a fuel gas and an oxidizing gas uniformly in inner planes of a cell;
b) a function of a channel which effectively discharges water formed in cathode portions to outside a fuel cell, along with a carrier gas such as air or oxygen after reaction;
c) a function of an electrical connector between membrane electrode assemblies so as to maintain for a prolonged period of time low resistance and high conductivity suitable for an electrode;
d) a function of a separator which separates a cathode chamber and an anode chamber in adjacent assemblies; and
e) a function of a separator which isolates cooling water channels and separates adjacent assemblies. In the present invention, the bipolar plate has at least the aforementioned function c).
The present inventors have conducted a variety of tests so as to develop stainless steel exhibiting low contact electrical resistance and excellent corrosion resistance, particularly stainless steel which exhibits no increase in contact electrical resistance with a graphite electrode material even when the steel serves as bipolar plates in a PEFC for a long period of time. The inventors have obtained the following findings.
a) Passive film formed on the surface of stainless steel inevitably exhibits electrical resistance. Therefore, when typical stainless steel covered with passive film serves as bipolar plates in a PEFC it is difficult to maintain the electrical resistance low so as to obtain sufficient cell performance.
b) Contact electrical resistance depends on the number of contact points per unit area; overall area of contact points; and electrical resistance of each contact point.
c) A conductive metallic inclusion of carbide and a conductive metallic inclusion of boride are dispersed and exposed such that the inclusions protrude to the surface of stainless steel from passive film, to thereby drastically lower contact electrical resistance and continuously maintain contact electrical resistance low. In this case, the conductive metallic inclusion of carbide and the conductive metallic inclusion of boride function as electric conduction paths.
d) When stainless steel is used in a PEFC, stainless steel exhibits relatively good corrosion resistance. However, metal elements are dissolved to result in corrosion, and corrosion products predominantly comprising iron hydroxide are formed, to thereby elevate contact electrical resistance and considerably affect a catalyst included in a fuel cell. Thus, cell performance represented by electromotive force is lowered within a short period of time, and proton conductivity of a proton-conductive fluoride ion-exchange membrane having a hydrogen-ion (proton)-exchange group is lowered.
e) In contrast, passive film is essential for assuring corrosion resistance of stainless steel within a PEFC. However, when the thickness of passive film is increased to thereby strengthen passive film, contact electrical resistance increases to thereby drastically lower cell efficiency.
f) In order to strengthen passive film and prevent release of metallic elements into a PEFC, the Cr content and Mo content (Cr%+3xc3x97 Mo%) is preferably controlled to 13 or higher.
g) When B is added to stainless steel so as to intentionally precipitate M2B metallic inclusions of boride, Cr, Mo, Fe, and Ni serving as corrosion-resistance-enhancing elements are consumed, to thereby drastically affect corrosion resistance of a steel matrix due to a reduction in Cr and Mo concentrations. Therefore, in order to elevate the Cr and Mo concentrations in steel for strengthening passive film and to prevent release of metallic elements into a PEFC, (Cr+3Moxe2x88x922.5B) is preferably controlled to 13 or higher.
h) Stainless steel in which Cr-based carbide is precipitated can continuously maintain low contact electrical resistance, regardless of the thickness of passive film. However, the amount of C contained in Cr-based carbide precipitates and the total C content in steel preferably satisfy the following conditions:
[(C mass % precipitated as Cr-containing carbide)xc3x97100/{(total C mass % in steel)xe2x88x920.0015%}]xe2x89xa780
for ferritic stainless steel, and
[(C mass % precipitated as Cr-containing carbide)xc3x97100/{(total C mass % in steel)xe2x88x920.012%}]xe2x89xa785
for austenitic stainless steel.
i) Intentional addition of Mo results in sufficient corrosion resistance. Even though Mo dissolves to a fuel cell, Mo exhibits relatively weak effects on performance of catalysts included in an anode and a cathode. The supposed mechanism is such that released Mo forms a molybdate ion, which does not prevent proton conduction of a proton-conductive fluoride-ion-exchange membrane having a hydrogen-ion (proton)-exchange group. Similarly, W forms a tungstate ion and exhibits an effect similar to that of Mo. Furthermore, V also exhibits a similar effect.
j) In order to expose conductive metallic inclusions which are precipitated in stainless steel, a method comprising dissolving the surface of stainless steel by use of an aqueous acidic solution is preferred.
k) If stainless steel which is washed with water and dried after completion of pickling is left as is, contact electrical resistance is liable to increase with elapse of time. This is predominantly caused by oxidation due to oxygen contained in the air. Specifically, an acid remaining in microcavities formed in the pickled surface is evaporated, concentrated, and gushes out, to thereby cause corrosion. The surface of stainless steel immediately after pickling has a very thin passive film, and water molecules forming a hydrate or hydroxynium ions are attached to the surface. When such stainless steel is allowed to stand in air, water molecules are dissociated and evaporated until attaining an equilibrium state. Oxygen is then bound to water-molecule-dissociated sites, to thereby deposit corrosion products and elevate contact electrical resistance. Since such a phenomenon proceeds within several hours, contact electrical resistance increases by the time stainless steel serves as electricity-conducting elements.
l) However, when pickled stainless steel is treated with an aqueous alkaline solution, deterioration of performance is remarkably mitigated; particularly, increase in contact electrical resistance in a corrosive environment, particularly in a bipolar plate.
m) The extent of exposure and protrusion height of conductive metallic inclusions determining contact electrical resistance can be industrially controlled by means of varying the average corrosion amount (dissolution amount). Furthermore, the average corrosion amount resides within an appropriate range in which contact resistance is minimized. This is related to the number of contact points and surface roughness, which determine contact electrical resistance.